A novel family of large GTPases (Mr=70,000-100,000) includes the following proteins: the interferon-inducible Mx proteins of vertebrates; the constitutive, microtubule-binding protein dynamin of Drosophila and vertebrates; the mitochondrial protein MGM1 of yeast, and the cytoplasmic protein VPS1/SPO15 of yeast. Despite striking sequence similarities, these proteins differ widely in their biological functions as defined by genetic models. The goal of our studies is to define more precisely the biochemical and functional parameters of the vertebrate members of this family of proteins. In some species, such as rats or mice, Mx proteins show activities against viruses that are of no importance to those species, suggesting the proteins' primary function may not be to inhibit viruses. In support of this suggestion, we have found that the anti-VSV protein Mx2 of rats binds microtubules in vitro in a nucleotide-dependent fashion similar to rat dynamin, Thus, Mx2 may have a role in cellular processes involving microtubules, and the antiviral action of Mx2 may merely be a byproduct of such a cellular role, the precise function of dynamin is not known for vertebrates but appears to be in endocytosis (for instance at the neuromuscular junction) in Drosophila, In order to genetically define the function of dynamin, we plan to generate a dynamin null-mutation in the mouse. To this end, we have isolated genomic cosmid and lambda phage clones corresponding to the mouse dynamin gene. Purification of rat Mx proteins derived from E. coli allowed us to initiate a detailed biochemical analysis of their enzymatic properties. The purified preparations also have been used to generate polyclonal rabbit antisera and to obtain initial structural information by low temperature, field emission scanning-transmission electron microscopy (Dr. Brian Andrews,LN). These will be useful to elucidate the mechanism by which viruses are inhibited.